Books and other similar media items are increasingly being delivered and consumed in electronic format, rather than on traditional paper or other physical media, for display on electronic viewing devices such as computers and mobile telephones. These viewing devices enable fast and low cost delivery of the electronic content via a network such as the Internet, mobile telephone networks, and so forth.
Digital rights management (DRM) is a critical component in the distribution of electronic books. Without effective DRM, authors and publishers would likely resist electronic distribution of their works. Indeed, electronic book distribution relies on an economic model that assumes effective DRM.
Existing DRM technologies applied to book distribution include encrypting the books using conventional symmetrical encryption techniques. Prior to distribution, a book is encrypted with a key that is either known to the consumer's reader device or can be somehow calculated or obtained by the reader device as needed. In order to prevent books from being freely shared between different reader devices, the encryption key is different for each reader device.
In one popular scheme, the encryption key is calculated as a function of the serial number of the device on which the book will be rendered. Before distributing a book for consumption on a particular reader device, a content server must know the serial number of the device. Assuming such knowledge, the server calculates the encryption key using a pre-defined algorithm that is a function of the device serial number, and encrypts the book using that encryption key.
After receiving the encrypted book, the reader device uses its serial number and the pre-defined algorithm to recalculate the encryption key, and uses that key as a decryption key to decrypt the book so that it can be rendered in an intelligible format to a user.